In conventional gasoline engines, particularly those used in the automotive industry, a carburetor mounted atop an intake manifold forms the principal component of a fuel system. As is well known, combustion air is drawn through the carburetor. A controlled amount of gasoline is added to the incoming air to form a combustible fuel/air mixture, as the air passes through a venturi throat formed in the carburetor. The intake manifold, which includes passages that communicate with valve controlled intake ports in the cylinder head of the engine, conveys and distributes the fuel/air mixture from the carburetor to the combustion chambers.
In theory, the liquid gasoline is vaporized prior to entering the combustion chambers. In practice, however, a major portion of the gasoline remains unvaporized and in a liquid state even as it enters the combustion chamber, finally vaporizing during the combustion process. The presence of unvaporized fuel in the combustion chamber, reduces the heat of combustion, thus limiting the power output of the engine.
It has long been recognized that the efficiency of the gasoline engine is substantially less than ideal. One factor contributing to poor efficiency in some engines is known as carburetor "double pull". Since the intake port is usually opened well before the exhaust stroke is completed, gases are forced in a reverse direction through the carburetor venturi drawing fuel into this flow. This reverse flow goes into the air inlet and filter wasting fuel. Another factor is a substantial portion of the energy available in each pound of gasoline consumed by an engine, is discharged to the ambient as waste heat from its cooling and exhaust systems and by way of radiation from the engine. Automotive designers over the years have proposed methods and apparatus for recapturing and utilizing at least a portion of this waste heat.
One proposed apparatus is an exhaust driven supercharger, more commonly called a turbocharger. A turbocharger generally comprises a pair of turbines mounted to a common shaft. One turbine is a drive turbine disposed in an exhaust flow path, while the other turbine is a compressor turbine disposed, at least in some instances, in the intake flow path between the carburetor and the combustion chambers. In this configuration, the exhaust gases discharged by the combustion chambers expand across the exhaust turbine to rotate it and the intake turbine thereby compressing gases in the fuel air mixture. This compression permits an increase in the amount of fuel introduced into each piston cylinder during the intake stroke of its piston while maintaining a desired fuel/air ratio, to produce an attendant increase in the engine's power output.
The addition of a turbocharger has not increased engine fuel efficiency in normal automotive usage. In general, the turbocharger allows a smaller engine with less friction to be used in a given size vehicle.
With these prior engines under operating conditions where cylinder intake produces high vacuum, pressure from the ambient air on the intake side of the turbocharger may exceed manifold pressure thus creating a pressure differential across the compressor side of the turbocharger. When an engine is idling there is little exhaust flow to drive the turbine and high vacuum manifold conditions exist so there is a large pressure differential across the compressor side of the turbocharger. This pressure differential causes an air flow through the compressor side. This air flow applies rotational forces to the compressor blade in opposition to the drive turbine.
Because exhaust flow is low, the air flow produced forces may be sufficient to cause reverse rotation of the compressor and will in any event prevent effective turbocharger operation. Thus, under light load the turbocharger is essentially inoperative and in fact may run backwards.
Another problem with each prior engine with a turbocharger between its carburetor and its intake manifold occurs on acceleration. When the throttle opening is increased, the quantity of liquid fuel droplets contained in the fuel/air mixture is increased virtually instantaneously but exhaust flow is not. This additional liquid fuel causes a significant increase in the load on the compressor turbine. Indeed in test racing engines the load increases on occasion, have been great enough to cause compressor turbine destruction. Since the driving force from exhaust gases is substantially constant the compressor is slowed by this load increase and the compressing action of the turbocharger is reduced. In time the increased fuel produces increased exhaust gases, causing the turbocharger to increase its speed and output. In sum, prior turbocharged engines have slow response to demands for power increases and will consume excessive fuel for a time whenever there is a significant increase in throttle opening. In fact, this excessive fuel consumption has made it difficult, if not impossible, for prior turbocharged engines to meet Environmental Protection Agency (EPA) standards if the turbocharger in fact operates during testing.
Other proposals for increasing the fuel efficiency of gasoline engines have included methods and apparatus for heating the fuel to aid vaporization. Prior proposals have suggested heating the fuel-air mixture by transferring heat from either the engine cooling system or the engine exhaust system. Problems associated with heating a fuel/air mixture as it travels to a combustion chamber, have long been recognized.
These problems include an increase in the temperature of the fuel mixture decreases the mixture density and causes a decrease in the volumetric efficiency of the engine for it decreases the amount of fuel drawn into each cylinder during an intake stroke. In addition, heating the fuel often causes a vapor lock condition in the fuel system which partially or completely blocks the flow of fuel into the intake flow path, degrading engine performance. To avoid vapor lock, many of the proposed fuel mixture heaters operate during engine warmup only and are turned off once the engine reaches its operating temperature. Further, prior hot vapor engine proposals have utilized storage chambers from which the fuel air mixture is modulated. Such a chamber is large and can be dangerous.
With prior engines, during engine warmup, vaporized fuel condensed on the interior walls of the intake manifold and other surfaces. Manifold and carburetor heating systems have been proposed to operate during engine warmup and were intended to solve or minimize this problem. While such proposals might improve conditions during warmup, the fuel still experienced as many as four phase changes as it travelled from the carburetor to the combustion chambers, even in an engine that had reached its operating temperature. Specifically, portions of the fuel entrained in the mixture flow shift between vapor and liquid states as the mixture travels through the engine intake system. Moreover, these phase change characteristics in a multi-cylinder engine are uneven varying from cylinder to cylinder and further varying with engine speed and load.
These phase changes contribute to the reduction in thermal efficiency in an engine due to: (1) the induction of some liquid fuel into the combustion chambers; (2) the nonuniform nature of the fuel-air mixture; and (3) substantial heat energy losses to the intake manifold and other components of the fuel system. These losses are substantial because gasoline, like all liquids, has a relatively high heat of vaporization.
The prior proposals for increasing the thermal efficiency of an engine have not recognized or addressed this problem. In most of the proposed systems, the fuel or fuel mixture was merely to be heated by either fluid from the engine cooling system or alternately by exhaust gases.
Combining a turbocharger with a fuel mixture heating apparatus has been proposed in the past. In one such proposal, the fuel charge would be heated by exhaust gases during part throttle operating conditions only. During full throttle conditions, the exhaust gases would be diverted to a turbocharger and the fuel mixture would go unheated, so that its density would be maximized.
It has also been found that many engine designers are of the opinion that the fuel mixture should be cooled after leaving a turbocharger or a supercharger. A cooling device commonly called an "intercooler" is disposed between the outlet of the supercharger and the combustion chambers. The purpose of the intercooler is to remove the heat generated as the mixture is compressed so that the fuel mixture density is increased. These seemingly conflicting proposals would indicate that confusion and uncertainty still exist in fuel system design theory.
The measure of success, however, in increasing the fuel efficiency of an internal combustion engine does not reside in the complexity or simplicity of the apparatus or the rigid adherence to long taught engine design principals, but in the increase in gasoline mileage and engine performance actually achieved in a given size engine.